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The United States Army wanted an accurate sniper rifle during the Vietnam War. The M14 was selected because of its accuracy, reliability, and the ability for a quick second shot. As a result, in 1969, the Rock Island Arsenal converted 1,435 National Match (target grade) M14 rifles by adding a Leatherwood 3–9x Adjustable Ranging Telescope (ART) telescopic sight and furnishing National Match grade ammunition. It was designated the XM21 until 1975, when it became the M21 (although it had been unofficially called the M21 since December 1969).

In standard military use, the M21/M25 use the same 10- or 20-round box magazines as the other members of the M14 family, and weighs 5.27 kg without the scope. The U.S. military never officially authorized or purchased magazines in any other capacity, although 5 and 10 round magazines are commercially available.

The XM21 Sniper Weapon System was used by the US Army in the Vietnam War, and saw limited action in military conflicts and operations in the late 1960s until the late 1980s. The M21 is currently in use with various U.S. military units in Iraq and Afghanistan. There are limited numbers in some Army National Guard units and in a few specialized active units such as the OPFOR units of the Joint Readiness Training Center.[1]

Springfield Armory, Inc. also manufactures variants of its M1A rifle called M21 Tactical Rifle and M25 White Feather™ Tactical/Carlos Hathcock rifle, which are based upon U.S. Armed Forces' M21 and M25 Sniper Weapons Systems but are slightly different, most notably they are fitted with a Picatinny rail to mount a scope[2].

Members of the 1st Battalion, 32nd Infantry during joint South Korea/U.S. Exercise Team Spirit '84. The soldiers are armed with an M21 sniper rifle (left) and an M16 rifle with M203 grenade launcher (right)